JOURNAL OF FOREST SCIENCE, 53, 2007 (5): 231–242
Determination of the pressing parameters of spruce
water-resistant plywood
J. Hrázský, P. Král
Faculty of Forestry and Wood Technology, Mendel University of Agriculture and Forestry Brno,
Brno, Czech Republic
Abstract: The paper summarizes results of operations aimed at determining the pressing parameters of spruce
water-resistant plywood and testing the suitability of particular constructions of plywood. Constant and variable parameters were determined. In pressed plywood sheets, the shearing strength of gluing according to EW 100 and the
coefficient of compressibility were determined. In pressing, heat transmission through the set of veneers was analyzed
and effects of the moisture of veneers on heat transmission were tested. The percentage of resin hardening was also
determined. Results were statistically analyzed. The dependence was determined of shearing strength, coefficient of
compressibility and heat transmission on changes in pressing parameters. Results of the study consist in the proposal
of pressing parameters for particular constructions of plywood.
Keywords: plywood; gluing strength; veneer; plywood construction; statistical evaluation
Since the beginning of human civilization, man
has used a renewable resource, wood raw material,
which is probably one of the most important products of the vegetable kingdom. Wood from felled
trees caused some problems to users and processors;
the most serious of the problems was the conversion of stems to forms suitable for construction or
other purposes. These problems induced the need of
new processing technologies for the manufacture of
wood-based materials, e.g. veneers, plywood, solid
glued products, agglomerated materials and other
wood composites.
Wood becomes a shortage material all over the
world; it must be imported for processing into industrial agglomerations and its price on world markets
increases. Diminishing raw material resources and
the shortage of natural wood lead to the increasing
use of new materials with generally better properties more suitable for industrial production. These
materials are characterized by large dimensions,
uniformity of mechanical properties and greater
resistance to external effects. Large-area materials
are produced by pressing usually under increased
temperatures from wood elements obtained by mechanical or other division.
According to the size of these detached parts it
is possible to distinguish main types of large-area
laminated materials:
– plywood materials,
– agglomerated materials.
Unlike particleboards and fibreboards plywood
materials maintain the appearance of natural wood.
By reason of high raw material requirements the
manufacture of veneers and plywood materials
shows considerable technical and technological
development. The shortage of wood raw material,
its decreasing quality and particularly its increasing
price force the manufacturers of veneers and plywood sheets to use newer and more modern technologies enabling effective valorization in processing
the wood raw material aimed at achieving higher
aesthetic properties of wood.
Wood is characterized by a number of valuable
properties explaining its broad and versatile use.
Wood represents a firm but light material showing
good heat insulation properties, it is able to tolerate
Supported by the Ministry of Education, Youth and Sports of the Czech Republic, Project No. MSM 6215648902.
J. FOR. SCI., 53, 2007 (5): 231–242
231
a considerable load and to damp vibrations. Wood
conducts the electrical current poorly not being liable to corrosion. It can be machined by means of
cutting tools and joined using glues, nails or screws.
Under dry conditions, it can be transported easily.
Wood is characterized by resonance properties.
Wood shows, however, its drawbacks which decrease possibilities of the full utilization of its advantages. Dimensions of the wood raw material are
given by the size of the stem and it is not possible
to manufacture the material of larger areas from
it without dividing it into smaller parts and then
binding these parts together to a large-area material.
Wood does not show sufficient hardness being anisotropic, i.e. it has different physical and mechanical
properties in various directions. With the change in
moisture content it changes its shape (dimensions)
and properties. Mechanical properties of various
tree species but also of the same species are different.
Solid wood is subject to rot and does not resist the
effect of high temperatures, fire and water. It includes
defects (knots, cracks etc.) worsening its physical
and mechanical properties.
The above-mentioned drawbacks can be eliminated to a great extent or removed by the physical/
mechanical and chemical processing of wood into
sheet and board materials. Larger dimensions can
be achieved through the manufacture of rotary-cut
veneers and their subsequent gluing to large-area
sheets. Higher hardness of wood can be achieved
by pressing under a high pressure. The restriction
of anisotropic properties can be achieved by gluing
veneer plies under a certain angle of wood fibres
(plywood, laminated wood, etc.) when properties
of wood in various directions of wood fibres are
equalized. Fire and rot resistance can be increased
by retardation and antiseptic treatment. Increase in
water-resistance can be achieved using phenol-formaldehyde or melamine-formaldehyde adhesives,
foils or special surface treatment.
Plywood materials have shown broad possibilities
of their use in various fields thanks to their physical
and mechanical properties.
– Furniture manufacture – decorative veneers are
designed for veneered elements for cabinet, table
and other furniture. Construction veneers are
used for shaped veneers for the manufacture of
seating furniture, for lamellas for the manufacture
of spring grids. These materials are used for the
manufacture of school furniture, decorative and
construction elements of the interior furniture,
etc.
– Wooden and combined constructions of dwelling
and utility character, particularly panels of prefab232
ricated houses, roof trusses, floors, partition walls,
posts, tie beams, TJI beams, etc.
– Engineering constructions – used in combination
with other materials, sarking, inner and outer facing, gables, stairs, etc.
– Formwork – system boarding, circular boarding,
supporting elements of boarding, etc.
– Floors in industrial and storage halls, floors of
elevated storeys, mezzanines, loading and arrival
platforms, floors of scaffoldings, etc.
– Door manufacture – constructions of door wings
including doorframe, ornamental veneering, door
jackets, etc.
– Land means of transport – lorries, railway wagons,
floors, walls and interiors of buses, trams, cars,
trailers, caravans, superstructures of cars, etc.
– In industry – transport platforms, work platforms
and tables, specially loaded parts, etc.
– Farm buildings – roofs, walls, inside lining, silos
for fodder, fertilizers and cereals, farm vehicles
– Road signs and billboards, traffic signs, etc.
– Ships and yachts – decks, interior equipment taking into account aesthetic, strength, fire-technical
properties, rescue boats, bridge boats, ships for
the transport of liquid nitrogen, tankers, cooling
transport premises
– Aircraft constructions – small sports, utility (e.g.
farm) and transport planes – construction parts,
sheathing, propellers
– Containers and packing for shipping particularly
sea transport, hygienic areas, storage and distribution, pallets, cases, barrels, trays, etc.
– Foot bridges, sports ground equipment
– Toys, gaming machines, shell mouldings for cutting forms, shells for skateboard, etc.
– Sporting goods and equipment – floors of sports
halls, winter stadiums, ping-pong tables, squash
halls, sports halls, platform constructions, etc.
– Musical instruments – pianos, organs, stringed
instruments, musical and loudspeaker boxes
– Stage and studio equipments with requirements
for acoustic properties
– Crematory coffins – some parts of construction
– Various special products such as plywood pipes,
laminated pressed wood for the manufacture of
foundry models, etc.
According to their construction plywood materials
are as follows: plywood (joinery, building, packaging,
aircraft and laminated wood), core boards (blockboards, veneer-faced boards), sandwich boards
(honeycomb panels, Jiko) and special materials
(Parallam, Microllam, Intrallam, LVL, Europly); according to their shape there are flat and form ones
(Hrázský, Král 2005a).
J. FOR. SCI., 53, 2007 (5): 231–242
They can also be distinguished according to main
quality characteristics, i.e. service life (for outdoor
use and for the use in the internal dry environment),
according to mechanical properties, surface appearance, methods of surface finish (unsanded, sanded,
surface treated, coated).
Factor affecting the quality of a glued joint
Wood density. Strength properties of a glued joint
increase with increasing density in the same kind of
wood (Eisner et al. 1966). The width of annual rings
in spruce is a good indicator for practice. Based on
the width we can derive and assess the density of
wood.
Earlywood and latewood. Studies carried out in
pine have shown that the gluing quality is influenced
whether it is a joint of “latewood-latewood” (L–L),
“earlywood-earlywood” (J–J) or J–L. The highest
strength was achieved in J–J.
Wood porosity. Shearing strength increases with
increasing volume weight and thus decreasing porosity.
The direction of wood fibres in a glued area. Suchsland (1987) found that the depth of the adhesive
penetration into wood was related to the length of
fibres and to an angle between the fibres and the
wood surface. The depth of penetration can be calculated. In spruce, the penetration increases with
the increasing spread of adhesives and increasing
angle of fibres.
Wood strength and swelling. Spruce plywood is not
affected by various water baths before a shearing
test to such an extent as beech plywood. It can be
explained by the lower strength of spruce wood and
smaller tangential swelling (Suchsland 1987). It
was demonstrated by the results of shearing strength
test after wetting for 1–72 hours at 20–100°C.
Effects of the content of natural resins. Wood of
our conifers contains resins consisting of resin acids
of a polar character. Therefore, it shows good affinity both to wood and to an adhesive (Eisner et al.
1966).
Surface quality according to Suchsland (1987):
– submicroscopic roughness (microfibrillar tissue
structure),
– microscopic roughness (dimensions of cells, tracheids),
– technical roughness (annual ring structure).
He found that:
– the adhesion area and strength increased with
increasing profile depth,
– the strength decreased from a certain profile
depth.
J. FOR. SCI., 53, 2007 (5): 231–242
The quality of rotary-cut veneers. The criteria of
veneer quality are as follows: tensile strength across
the fibre, mean depth of cracks and the frequency
of cracks. In good-quality veneers, lower spread of
adhesives was achieved at a higher strength of gluing
(Král 2006).
Left and right side of veneers. Gluing tests in Norway birch demonstrated the best results in gluing “left
side-left side”, good results in “right side-left side” and
bad results in “right side-right side”. In spruce, no differences were found out (Kollmann 1975).
Veneer thickness. Veneers up to a thickness of
3.2 mm can be glued directly (Kollmann 1975).
In thicker veneers, planing is suitable before gluing;
a glued joint between two thinner veneer sheets is
more often defective than a glued joint between a
thin and a thick veneer sheet. With the increasing
thickness of veneers the spread of a gluing mixture
also increases.
Material moisture. Phenol-formaldehyde (PF)
adhesives are extremely sensitive to the amount of
moisture contained in veneers. Increased moisture
extends the pressing period, increases the compression of plywood and decreases the viscosity of an
adhesive spread, which results in deep penetration
into wood and joints of poor quality.
Total moisture. Moisture conditions are very important for the quality of gluing. Through gluing,
we bring further moisture into wood. After gluing,
the actually existing moisture appears to be the sum
of wood moisture before gluing and the moisture
increment. In connection with the initial moisture of
veneers and the amount of spread it usually ranges
between 8 and 15%. Under conditions of higher
moisture, damage to a glued joint can occur (open
joints or blisters).
Temperature of veneers and adhesives. The initial
temperature of veneers should be higher than or at
least equal to the temperature of an adhesive. Under
these conditions, vacuum is created in surface pores
due to a rapid decrease in the air volume caused by
a difference in temperatures, which supports the
penetration of the adhesive to a greater depth and
thereby the surface of a glued joint and mechanical
adhesion increase. At a lower temperature of veneers
than that of the adhesive a reverse process takes
place when the volume of air from pores increases,
small bubbles are created and then the air penetrates
under pressure through the adhesive layer decreasing its adhesion and thus also the gluing strength.
The greater the temperature difference, the more
important the effect.
Adhesive reactivity is the rate of resole conversion to three-dimensional molecules, i.e. the
233
condition of resite as a result of heat. The reactivity
of PF adhesives can be increased by the addition of
resorcinol, tanstuffs, hexamethylenetetramine etc.
Sedliačik (1995) reduced the period of pressing in
F – 20 adhesive by 1 min using a mimosa mixture
at 150°C.
Adhesive viscosity is an important indicator of glue.
An optimum viscosity for veneers thinner than 1.5 mm
is 2,000–2,500 MPa/s and for veneers over 1.5 mm
2,500–4,000 MPa/s.
Adhesive dry matter. It is known that with increasing dry matter the rate of adhesive hardening also
increases. At a temperature of 140°C and the dry
matter difference 11% (58–47%) a difference in the
time of hardening amounts to about 3 min. In this
country, the PF adhesive dry matter ranges mostly
about 50%. It is substantially lower abroad but extenders, fillers and other admixtures are used more
frequently there.
Adhesive spread. The amount of gluing mixture
spread in g/m2 exerts a great effect on the strength
of the glued joint.
Glued joint thickness. The layer of a spread gluing
mixture dries up in the process of hardening reducing
its volume, and thus stresses occur in the adhesive
layer. Volume changes in phenolic adhesives amount
to only 25%. There is an effort to preserve the minimum thickness of a glued joint ensuring the physical
and mechanical properties of the joint.
Surface stress. Maximum shearing strength and
maximum percentage of wood failure were demonstrated at surface stresses of 68.0 and 68.8 mN/m.
Contact angle. An increase in the quality of gluing
with an increasing contact angle was noted, which
contradicted a general view that a small contact angle was desirable. However, an interaction between
the properties of wood and adhesive shows considerable effects.
Resin alkalinity. Shearing strength and percentage
of wood failure were highest at pH 11. The value was
largely dependent on the amount of NaOH in the
reaction mixture.
Free phenol. With the decreasing content of free
phenol the PF adhesive reactivity increases.
Pressing diagrams. At the beginning, they were
simple. After reaching a certain value, the working
pressure was maintained for a certain time, and it
was decreased at the end of the pressing period. It is
recommended to reduce the pressure in two stages: in
the first from a maximum to 0.3–0.4 MPa and in the
second from 0.3–0.4 MPa to zero. The duration of the
first stage is 0.15–0.25 min. The duration of the second stage is dependent on the type of adhesive, pressing temperature, moisture content of the material and
234
number of layers. For PF adhesives and moisture content of veneers up to 12%, 35–45 s is mentioned in
three-plied plywood (birch, pine) at a temperature
of 135–155°C and 80–90 s in multilayer plywood
at a temperature of 135–137°C. The introduction
of the manufacture of multilayer plywood brought
about also changes in pressing diagrams. Multistage
pressing programmes have been introduced abroad.
Working pressures are decreased in several stages
there. This procedure has brought about a decrease
in the percentage of compression and the quality of
production has been improved.
Specific pressure. It should ensure the good contact
of glued surfaces. The rate of the used pressure is
primarily dependent on the pressed woody species.
Kafka (1989) mentions 1 MPa for softwood plywood, 1.2 MPa for combined plywood and 1.5 MPa
for hardwood plywood.
Pressing temperature. For the full thermic hardening
of phenolic resins a temperature of about 180°C is required. Temperatures 130–160°C used in practice give
joints resistant to water although the resin has not fully
hardened yet, the degree of hardening being, however,
sufficient for quality bonding. Multi-component PF
adhesives allow to decrease pressing temperatures
to 120–125°C, e.g. Finnish adhesives Vatex-224 and
Exter A. An addition of quebracho extract to phenol
adhesives has been introduced in Finland.
Pressing period. The pressing period of laminated
materials is dependent on many factors, the most
important of them being the temperature of compression plates, working pressure, woody species,
thickness of elements and of the product, kind of
resin and viscosity of its solution etc. From the
aspect of heat passage and temperature increment,
it is important to differentiate the plate margins.
The margin is a belt 7.5–10 cm wide along the plate
periphery. Heat passage is slower there mainly at
temperatures over 100°C (evaporation, heat dissipation). Calculation relationships are generally effective only for the central zones of plates not taking
into account KS. Because calculation relationships
do not provide the guarantee of sufficiently accurate
results, the measurement of temperature by means
of thermocouples is generally used in sets of veneers
to determine the temperature increment (and thus
pressing periods) (Šteller 1995).
Objectives of the study
Problems of pressing plywood are rather complicated. Based on the results of research and practical operations new factors are found. The aim of
the study was to determine pressing parameters of
J. FOR. SCI., 53, 2007 (5): 231–242
spruce water-resistant plywood for general use and
to test the suitability of particular plywood constructions.
Results are applied immediately in practice, and
thus we start from some practical data (e.g. tree
species, veneer moisture, amount of spread, initial
pressing periods, etc.). Possibilities were tested
to intensify the process of pressing by changes in
pressing temperature, pressing period and pressing
diagrams.
The quality of gluing and the actual plywood thickness were decisive for assessing the suitability of
some technological procedures of pressing. Results
were evaluated statistically.
Basic operations were completed by the measurement of heat passage, determining the percentage of
resin hardening, coefficient of compressibility, etc.
Results of the study consist in determining pressing
parameters for specific pressures of spruce water-resistant plywood. These results are used in practice.
Material and methods
In the experimental part of the paper, constant
parameters were determined first and then variable
parameters and the range of selection.
Constant parameters:
– PF resin P 5250; spread 150 g/m2,
– spruce veneer 1.8 mm thick; moisture 5 ± 2%,
– plywood structure.
Nominal thickness (mm)
Number of layers
5
8
12
15
18
3
5
7
9
11
Variable parameters:
– pressing temperature 160°C; specific pressures 1,
1.2, 1.4 MPa,
– pressing temperature 150°C; specific pressure
1 MPa and pressing diagrams A, B, C
– – (A) 1.2 MPa 50% pressing period; 0.8 MPa 50%
pressing period – 1 min; 0.4 MPa 1 min
– – (B) 1.4 MPa 1/3 pressing period; 0.7 MPa
1/3 pressing period; 0.2 MPa 1/3 pressing
period
– – (C) 1.2 MPa 50% pressing period; 0.6 MPa
50% pressing period – 1 min; 0.1 MPa –
1 min,
– pressing period – the starting period for each
thickness was a pressing period designated by t
used in practice. Next two pressing periods were
J. FOR. SCI., 53, 2007 (5): 231–242
determined as follows: t1 = t – 1' and t2 = t – 2'.
In construction 11×, at 150°C only time t was used
with respect to the results found at 160°C.
The extent of the test
Gluing quality is crucial to assess pressing parameters. In the basic series of tests, 254 sheets were
pressed in the laboratory and 2,520 specimens were
tested (shearing strength in gluing) according to EN
314. The specimens were air-conditioned and exposed to EW-100 (gluing class 3) before the test.
Moreover, the coefficient of plywood compressibility, heat passage and the percentage of resin hardening were determined (Hrázský, Král 2005a).
RESULTS AND DISCUSSION
The course of a temperature increment in the last
glued joint was determined by a copper-constantan
thermocouple. At the thermocouple gradation and
the actual measurement a connection was used with
a comparative end.
Pressing temperature 160°C
The results of heat passage measurement show that
an anticipated temperature of 130°C was achieved in
constructions 3×, 5× and 7× at all working pressures
earlier than in the shortest pressing period t2. In construction 9×, a temperature of 130°C was achieved in
all three pressing periods at P3 pressure. At P2 pressure, a temperature of 126–129°C was achieved
within the limits of t, t1, t2 pressing periods and at P1
pressure, a temperature of 121–126°C was achieved
within the limits of t, t1, t2 pressing periods. In construction 11×, a temperature of 130°C was achieved
at P3 pressure earlier than after the shortest time t2.
At P2 pressure, a temperature of 130°C was achieved
at t (at t1 – 129°C and at t2 – 127°C), P1 pressure was
achieved at t – 123°C, t1 – 121°C, t2 – 120°C. Of all
pressed sheets, only sheets 11× did not meet under
the following combinations of pressing parameters:
P1 – t1 (121°C), P1 – t2 (120°C), P2 – t2 (127°C). The
rate of heat passage increases with increasing pressure, which is mainly related to a reduction in the effective depth of warming-through due to the greater
compression of the set of veneers.
Pressing temperature 150°C
The anticipated temperature of 130°C at P1 pressure in plywood constructions 3×, 5×, 7× was
achieved in all three pressing periods t, t1 and t2. In
plywood construction 9×, the following temperatures
were achieved in the particular periods: t – 123°C,
t1 – 121°C, t2 – 118°C. In plywood construction 11×,
only period t was used and a temperature of 117°C
235
Table 1. Comparison of a theoretically calculated time necessary to achieve 130°C in the last glued joint (time in seconds)
Plywood
construction
Virtually determined time
Theoretically calculated time
160°C
160°C
150°C
P1
150°C
P2
P3
P1
3×
41
52
90
60
42
126
5×
134
165
144
114
84
270
7×
271
338
228
180
156
252
9×
458
556
–
612
456
–
11 ×
696
843
–
720
462
–
was achieved. All pressed sheets complied with
requirements (a shearing test in the plane of the
plywood complies with EV/-100). Comparison of the
theoretically calculated time necessary to achieve
130°C in the last glued joint and results of practical
measurements are given in Table 1.
Heat passage through the set of veneers
of higher moisture
Heat passage was studied in plywood sets 11 ×
1.8 mm, format 250 × 250 mm at the veneer moisture
of 10 ± 2%. Other parameters were as follows: 150°C;
1 MPa; pressing period – after achieving 130°C in
the last glued joint; adhesive spread 150 g/m2. Heat
passage at W = 10% is much slower than at W = 5%.
A temperature of 117°C was reached after 12 min.
W 5% was reached after 18 min and a temperature
of 130°C after 22 min. Slowdown was noted particularly after exceeding 100°C. This temperature was attained at both veneer moistures after about 5 min, of
course, in veneers W = 10% a temperature of 108°C
was achieved after 12 min and 117°C after 18 min.
Pressing period is extended by 50%. The temperature
inside a bundle increases more slowly although the
air heat conductivity is almost 23 times lower than
the heat conductivity of water. During the gradual
warming-through of the set of veneers a woody
species changes its temperature together with water
contained in wood and adhesive. At a temperature
of 100°C, a part of moisture changes into vapour
filling all spaces in wood and between veneer sheets.
If there is no passage of vapours from the centre of
the set pressed between pressing plates, then the
vapour more and more increases its temperature,
which increases the inner stress of vapours in the
veneer set. Resistance of the vapour passage through
any duct is directly proportional to the length of the
path the vapour has to pass. Vapour from marginal
parts of the veneer set passes to the ambient air and
the vapour pressure decreases. Due to the pressure decrease evaporation stops at the expense of
superheated water in the boiling point decreasing
again during evaporation, which becomes evident
particularly intensively as soon as a temperature of
100°C is achieved. Due to this evaporation partial
losses of evaporation heat will occur. According to
drying the marginal parts of the veneer set this process will transfer to the centre of the set, resistance
for the vapour passage will increase, the amount of
evaporated water and heat losses will decrease, and
thus the temperature in the marginal part of the set
will increase. The temperature of the central zone of
the set at larger formats continuously increases to
the temperature of pressing plates.
It has been found that in construction 3×, a temperature of 125–130°C in the last glued joint does
not suffice to obtain a water-resistant joint after
108 s. In construction 11×, results demonstrated that
temperatures 115, 117 and 120°C were sufficient for
the creation of a water-resistant joint. Based on the
results mentioned above it is possible to conclude
that not only a certain temperature but also a certain
time interval of the effect of the given temperature
are necessary for the water-resistant hardening of a
glued joint.
Table 2. The proportion of the veneer set thickness per glued joint
Construction
3×
5×
7×
9×
11×
Gross thickness of plywood (mm)
5.40
9.00
12.60
16.20
19.80
Number of glued joints
2.00
4.00
6.00
8.00
10.00
Thickness per 1 glued joint (mm)
2.70
2.25
2.10
2.03
1.98
11.50
12.80
13.30
13.60
13.80
Moisture (%)
236
J. FOR. SCI., 53, 2007 (5): 231–242
It is possible to conclude:
– the rate of heat passage increases with the increasing working pressure (within a certain pressing
temperature),
– the rate of heat passage decreases with the increasing moisture of the veneer set,
– to obtain water-resistant joints not only a certain
temperature but also a certain interval of the effect
of a certain temperature are necessary.
Coefficient of compressibility
In all pressed sheets, the coefficient of compressibility was determined. The values KS given thereinafter represent a mean of six values (160°C) and
of three values (150°C). The actual thickness of
plywood given in an attached table is an important
indicator.
The ČSN EN 635 standard prescribes parameters
of allowable deviations for water-resistant plywood
of gluing class 3.
Pressing temperature 160°C
The results of the analysis of variance inside
particular constructions of plywood under P 1,
P2, P3 pressures showed that a difference between
variances was significant in all constructions. The
same result was found in the analysis of variance
between particular constructions of plywood at P1
pressure. Testing the differences in arithmetical
means demonstrated that there was a significant
difference between particular means. Also testing
the differences in arithmetical means at P1 pressure
between various constructions of plywood shows a
statistically significant difference in the following
combinations of plywood constructions at P1 pressure (at a = 95% and a = 99%):
3×–9×
5×–9×
7×–9×
3×–11×
5×–11 ×
7×–11×
Certain regularity occurs there, namely plywood
up to the number of 7 plies is compressed less than
plywood 9× and 11×. Causes of these differences are
elucidated in Table 2.
Table 2 shows that in veneer sets according to the
number of plies we obtain different thickness of a veneer set corresponding to a glued joint and different
aggregate moisture if we preserve constant conditions
of production. These differences condition various
values of the coefficient of compressibility.
Pressing temperature 150°C
Analysis of variance between particular constructions of plywood at P1 pressure showed that differences were significant. Testing the differences in
J. FOR. SCI., 53, 2007 (5): 231–242
arithmetical means at P1 pressure between various
constructions of plywood did not bring so marked
differences as at a pressing temperature of 160°C. On
the significance level a = 95%, differences were found
in the following constructions of plywood: 3×–7×,
3×–9×, 3×–11×, 5×–9×.
Due to the small number of measurements stepped
pressing diagrams were not statistically evaluated.
According to mean KS it is possible to propose the
following diagrams in particular constructions of
plywood: 3× – C, 5× – A, 7× – A, 9× – C, 11× – A.
To achieve the given coefficient of compressibility
plywood 7× (150°C, P1, t) was pressed. The aim was
to achieve the given value KS 5%. Veneer sets were
inserted into a press on the plates of which a thickness meter was fixed. Then the pressure was applied.
After achieving the calculated value KS the pressure
was reduced in order not to increase the compression.
Mean KS = 6.45% was achieved as against KS 10.07%
(160°C, P1) and KS = 9.79 (150°C, P1).
Effects of the moisture of veneers on the coefficient
of compressibility. It was noted in the set 11 × 1.8
(150°C, P1, 12 min, W = 10%). Mean KS = 13.97%, i.e.
an increase by 3.81% as against W = 5%.
Effects of a specific pressure on the coefficient of
compressibility. Analysis of variance within particular plywood constructions at P1, P2 and P3 pressures
and 160°C showed that differences were statistically
significant in all constructions.
Results of the analysis of variance between particular constructions of plywood at P1 pressure and
temperatures 150°C and 160°C showed statistically
significant differences as well.
Testing the difference in arithmetical means was
carried out within particular plywood constructions at P1, P2 and P3 (160°C) – differences were
statistically significant.
Results of testing the differences in arithmetical
means at P1 pressure are given in paragraphs relating to
pressing temperatures. It is possible to conclude that:
– differences between coefficients of compressibility at working pressures P1, P2 and P3 (160°C) are
statistically significant,
– differences between arithmetical means of coefficients of compressibility of various plywood
constructions at P1 pressure are statistically significant more frequently at a higher temperature
(160°C) than at a lower temperature (150°C),
– stepped pressing diagrams improve the quality of
production (smaller number of vapour blisters)
and reduce the coefficient of compressibility (in
plywood 9× and 11×),
– the coefficient of compressibility can be intentionally controlled.
237
238
J. FOR. SCI., 53, 2007 (5): 231–242
P2
P1
9.63
9.50
9.60
9.58
Specific pressure
Pressing period t
Pressing period t1
Pressing period t2
Mean KS
16.16
16.38
15.35
15.76
P3
9.63
9.85
9.51
9.70
P1
12.74
12.61
12.70
12.92
P2
5×
17.56
16.82
17.15
18.71
P3
P1
8.77
9.27
8.11
8.72
Specific pressure
or regime
Pressing
period t
Pressing
period t1
Pressing
period t2
Mean KS
Plywood
construction
11.31
A
3×
14.96
B
11.01
C
9.26
8.60
10.10
9.07
P1
11.33
A
5×
12.50
B
14.41
C
9.79
9.72
10.20
9.45
P1
9.91
P1
9.94
A
10.07
10.22
10.08
Table 4. Mean values of coefficients of compressibility at a pressing temperature of (150 ± 3)°C
12.48
12.54
13.03
11.80
3×
Plywood construction
Table 3. Mean values of coefficients of compressibility at a pressing temperature of (160 ± 3)°C
7×
16.00
B
12.48
12.72
12.31
12.40
P2
7×
10.65
C
19.62
19.54
18.92
20.41
P3
10.47
9.65
11.45
10.30
P1
A
15.22
11.31
10.71
18.28
10.95
P1
9×
11.33
B
15.02
14.93
15.49
14.63
P2
9×
8.87
C
19.75
19.55
20.44
19.25
P3
10.16
8.77
11.97
9.74
P1
11.53
11.09
11.46
12.05
P1
9.22
A
11×
9.93
B
14.93
15.14
14.54
15.23
P2
11×
12.60
C
21.05
19.29
20.91
22.94
P3
J. FOR. SCI., 53, 2007 (5): 231–242
239
Pressing
period t2
H
Σhi
H
4.82
4.98
5.46
4.96
5.48
5.00
4.85
5.54
4.87
5.57
4.86
5.59
4.99
5.37
5.47
5.52
4.66
5.55
4.68
5.59
4.70
5.52
4.61
5.54
P3
8.34
9.23
8.47
9.39
8.36
9.23
8.20
9.09
P1
8.09
9.29
8.13
9.32
8.15
9.34
7.99
9.18
P2
5×
7.69
9.33
7.70
9.25
7.73
9.33
7.65
9.41
P3
11.56
12.85
11.58
12.88
11.51
12.80
11.59
12.86
P1
11.29
12.88
11.23
12.83
11.37
12.96
11.27
12.86
P2
7×
Pressing
period t2
Mean
Σhi
Pressing
period t1
H
Σhi
H
H
Σhi
H
Σhi
Pressing
period t
Specific pressure
or regime
Plywood
construction
4.86
5.20
5.13
5.62
5.10
5.55
5.09
5.61
4.49
A
5.70
P1
3×
4.77
5.60
B
4.94
5.55
C
8.62
9.50
8.71
9.53
8.63
9.60
8.52
9.37
P1
8.19
9.22
A
5×
8.05
9.21
B
7.60
9.20
C
11.42
12.55
11.54
12.89
11.18
12.45
11.43
12.62
P1
11.14
12.43
A
7×
B
11.04
13.14
Table 6. Mean thickness of veneer sets and plywood at a pressing temperature of (150 ± 3)°C (mm)
Mean
Σhi
Pressing
period t1
H
Σhi
H
Σhi
Pressing
period t
P2
Specific
pressure
P1
3×
Plywood
construction
Table 5. Mean thickness of veneer sets and plywood at a pressing temperature of (160 ± 3)°C (mm)
11.15
12.46
C
10.30
12.91
10.32
12.83
10.52
12.98
10.29
12.93
P3
14.63
16.33
14.79
16.37
14.57
16.45
14.52
16.18
P1
14.69
16.25
14.70
16.47
14.68
16.74
14.70
16.52
P1
13.98
16.49
A
9×
B
14.55
16.41
14.12
16.64
14.13
16.58
14.08
16.77
14.15
16.58
P2
9×
14.78
16.22
C
13.67
17.03
13.68
17.00
13.54
17.01
13.79
17.08
P3
17.77
19.93
18.30
20.06
17.36
19.72
18.07
20.02
P1
18.03
20.37
18.15
20.41
18.08
20.42
17.85
20.29
P1
18.19
20.04
A
B
18.01
20.01
11×
17.36
20.58
17.30
20.38
17.43
20.89
17.36
20.48
P2
11×
18.17
20.79
C
16.00
20.26
16.29
20.16
16.12
20.38
15.59
20.22
P3
Mean values of coefficients of compressibility at
various combinations of pressing parameters are
given in Tables 3 and 4.
Mean thickness of veneer sets and plywood is
given in Tables 5 and 6.
– at a pressing temperature of 150°C, a difference
occurred between variances only in sheets L in
construction 5×,
– in stepped pressing diagrams, a decrease in
shearing strength was found in sheets P in constructions 3× and 5× (the values markedly exceed
requirements of standards),
– designed pressing parameters based on the results
of shearing tests: pressing temperature 150°C and
subsequently in particular plywood constructions: 3× (P1–t2 or C–t2), 5× (A–t2), 7× (A–t2), 9×
(C–t2), 11× (A–t).
Shearing strength on the level of plywood layers
Pressing temperature 160°C
Remaining pressing parameters P1, P2, P3 and t, t 1
t2. During cutting the specimen sheets 11× disintegrated in combinations P1 – t1, P1 – t2, P2 – t2. In
the remaining sheets, higher strength was achieved
than required by the standard. Results of one- and
two-factor analysis of variance within particular
constructions demonstrated that differences were
statistically insignificant in all cases.
Determination of the percentage
of PF resin hardening
The aim of studying the character of PF resins in the
field of UV spectrum was to determine the amount
of the insoluble proportion of a heat-hardened resin.
Methods according to Chow (1967) should be used.
However, the characteristics of extinction curves
obtained in P 5250 did not make it possible to use the
methodology. Therefore, evaluation of the insoluble
proportion was carried out from extinctions measured at λ max. = 283 nm. Spectral analyses were carried out in water solutions using a Specord UV-VIS
apparatus of Zeiss Co.
The following measurements were carried out:
– dependence was found of the concentration of adhesive water solutions on extinction at 283 nm,
– thermal condensation of a PF adhesive was carried
out at 130, 150, 170°C for various times,
– measured values served for the calculation of regression lines of the dependence of the degree of
hardening, solubility and extinction; plywood 3 ×
was pressed, pressing parameters (1.2 MPa; 130°C,
150°C, 170°C), 1'48'', 2'48'', 4'48'', 6'48''),
– dry shearing strength was determined in samples
from the plywood and after AW-100 test, the percentage of resin hardening was assessed in a glued
joint,
– the values of extinctions and corresponding
percentage of resin hardening are given in Table 7.
Pressing temperature 150°C
Plywood pressed in the laboratory (P1; t, t1, t2, in
construction 11× only t). Plywood pressed in operation
(P1 and pressing diagrams C, A, A, C, A – arranged in
the order of an increasing number of plywood plies;
pressing period t, t1, t2 (at P1) and t2 in stepped pressing
diagrams – in construction 11× only time t).
One-factor analysis of variance was carried out
for plywood pressed in the laboratory (L) and in
operation (P). Wood is a markedly heterogeneous
material, and therefore it is possible to consider a
difference between variances to be significant only if
F = 0.01. Then, a difference from plywood sheets L in
construction 5× and from sheets P in constructions
3× and 5× is significant.
Results of testing the difference in arithmetical
means: in sheets L, a statistically significant difference
was found in construction 5× (t–t2 and t1–t2) and in
construction 9× (t–t1,). In sheets P, these statistically
significant differences were found: 3× (t–t1; t1–t2; t–Ct2;
t2–Ct2), 5× (t–t1; t–2; t–At2, t1–At2) and 7× (t–t1).
Summary
– at a pressing temperature of 160°C, only accidental difference was found between variances
in all plywood constructions (except 11×),
Table 7. Values of extinctions and the percentage of resin hardening
Pressing temperature
Pressing time
130°C
E
150°C
(%)
E
170°C
(%)
1'48''
E
(%)
0.31
57
2'48''
0.87
0
0.31
51
0.28
60
4'48''
0.35
52
0.25
59
0.21
69
6'48''
0.26
62
0.21
64
0.17
74
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J. FOR. SCI., 53, 2007 (5): 231–242
Summary
– effects of extractive substances and veneers
were measured, however, they did not show any
effect on the actual measurements of extinctions of extracts from resins giving a zero value
to extinction; the presence of wood does not
affect measurements of the percentage of resin
hardening,
– results of shearing strength tests and the percentage of hardening the PF resin in a glued joint
show that the joint is sufficiently water-resistant
at hardening the adhesive exceeding 51% determined according to the methodology,
– in plywood 3 ×, the joint was water-resistant after pressing periods: 30°C – 4'48'', 150°C – 2'48'',
170°C – 1'.
conclusion
Results of the study are used in a particular plant
in the large-scale production of coniferous water-resistant plywood. For the first time, multistage pressing diagrams of a new type were used. A method has
been proposed and experimentally tested making it
possible to decrease the coefficient of compressibility
in multiply plywood and, at the same time, to reduce
the occurrence of vapour blisters.
Through experimental measurements, the whole
production spectrum of plywood constructions was
proved in relation to their actual thickness.
Determination of the percentage of resin hardening demonstrated that the dry matter of a gluing
mixture was not used effectively. It is necessary
to use multi-component gluing mixtures with the
reduced dry matter of an actual resin and to utilize
fillers and extenders. It will result in the reduction
of production costs and better technologies. Determination of pressing parameters and testing
plywood constructions occurred simultaneously
with the determination of prepressing parameters.
summary
The paper objective was to determine pressing
parameters of spruce water-resistant plywood for
general use and to test the suitability of actual plywood constructions.
The selected method of processing resulted from
the scheduled objective of the study. Constant (PF
resin F 5250, spread 150 g/m2, spruce veneer 1.8 mm
thick, veneer moisture 5%, plywood construction)
and variable parameters were determined (pressJ. FOR. SCI., 53, 2007 (5): 231–242
ing temperature, working pressure, pressing period,
pressing diagrams). The following parameters were
determined in pressed sheets: shearing strength of
glued joints after EW-100 exposition and coefficient
of compressibility. During pressing, heat passage
through the set of veneers was studied. Other tests
were also carried out, namely determination of the
percentage of resin hardening, intentional control
of the coefficient of compressibility, effects of veneer
moisture on heat passage and coefficient of compressibility etc. The extent of basic tests was determined statistically and the results were also evaluated
statistically.
The paper results in the proposal of pressing
parameters for actual plywood constructions. Relationships were also determined between shearing
strength, coefficient of compressibility and heat
passage and changes in pressing parameters. Determination of the percentage of resin hardening
has demonstrated that the adhesive dry matter is
not used economically at present and, therefore,
it is necessary to introduce multi-component gluing mixtures. Results of the study are applied in
practice.
References
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Received for publication October 25, 2006
Accepted after corrections January 8, 2007
241
Stanovení lisovacích parametrů smrkových vodovzdorných překližek
Abstrakt: Článek shrnuje výsledky práce, jejímž cílem bylo stanovit lisovací parametry smrkových vodovzdorných překližek a ověřit vhodnost jednotlivých konstrukcí překližek. Byly určeny konstantní a proměnné parametry.
Na vylisovaných překližovaných deskách byla stanovena pevnost lepení ve smyku podle EW 100 a koeficient slisovatelnosti. Při lisování byl analyzován prostup tepla dýhovým souborem a ověřen vliv vlhkosti dýh na prostup tepla.
Rovněž bylo stanoveno procento vytvrzení pryskyřice. Výsledky byly statisticky vyhodnoceny. Byla stanovena závislost
smykové pevnosti, koeficientu slisovatelnosti a prostupu tepla na změnách lisovacích parametrů. Výsledkem práce
je návrh lisovacích parametrů pro konkrétní konstrukce překližek.
Klíčová slova: překližka; pevnost lepení; dýha; konstrukce překližky; statistické vyhodnocení
Corresponding author:
Doc. Dr. Ing. Jaroslav Hrázský, Mendelova zemědělská a lesnická univerzita, Lesnická a dřevařská fakulta,
Lesnická 37, 613 00 Brno, Česká republika
tel.: + 420 545 134 159, fax: + 420 545 211 422, e-mail: [email protected]
242
J. FOR. SCI., 53, 2007 (5): 231–242